1. Field of the Invention
This invention relates to an antenna for wireless radio communication which receives VHF band and UHF band and, more particularly, to an antenna with decreased visibility of itself and a method of making the antenna.
2. Description of the Related Art
Conventionally, in considering a half-wave dipole antenna as an antenna element to send and receive VHF band (30-300 MHz) and UHF band (300 MHz-3 GHz), a dipole antenna 3 comprises a pair of conductor plates 1, 1, and a feeding portion 2 connected with the conductor plates 1, 1 as shown in FIG. 8.
To decrease the visibility, a film antenna is known that the conductor plate 1 is composed of printing with conductive paste or wire material. Although length L may be varied, the most fundamental one is provided with a length of ½ of the wavelength, for instance, L=about 300 mm for 500 MHz because its wavelength is 600 mm. In this case, although the width W of the conductor is determined by a resistance value for impedance matching with the feeding portion, it needs generally to be several mm or more as a practical size even if the copper wire with a low resistivity is used. Therefore, the visibility cannot be decreased.
In case of using the conductive paste, the resistivity is about 20 times compared with copper even if the paste is of silver having the lowest resistivity. Thus, if the conductor width is not wider than that in case of using the copper wire, the resistance increases and the impedance matching with the feeding portion cannot be obtained.
Conventional antennas have been manufactured by the following methods.
(1) An antenna is formed by passing a conductive thin wire through a special tool (nozzle), moving on the orbit of the special tool while discharging the thin wire, and pasting the thin wire on an adhesive sheet (called drawing method, e.g., JP-A-2000-76398).
(2) An antenna is formed by preparing a substrate, screen-printing a conductive ink thereon by using a mesh plate, and drying and hardening the ink (called paint method, e.g., JP-A-2001-102745).
(3) A coil is formed by using a metallic foil as a conductor, masking a part to be left as an antenna, and removing other part than the part to be left by etching (called etching method, e.g., JP-A-2001-101371).
As a result, the conventional antenna element is not negligible in visual sense since the conductor plate 1 is as large as several mm in width W and about 300 mm in length L. When the antenna is installed at a place such as a window of a vehicle, inside of the vehicle or on the periphery of a television, the visibility in seeing the outside from the vehicle may be reduced, or the design harmony of the entire vehicle may deteriorate.
Further, there are following problems in view of the manufacturing method of the antenna.
In the above drawing method (1), since it is difficult to increase the density, the entire antenna area must be too large in case of using many turns. Further, when manufacturing an antenna in complex shape, it takes time since the drawing work is not easy, and the cost rises that much.
In the above paint method (2), the resistivity is generally about 20 times compared with the copper even if the paste is silver having the lowest resistivity. Thus, if the conductor width is not wider than that in case of using the copper wire, the resistance increases and the impedance matching with the feeding portion cannot be obtained. Further, the cost is increased due to using the expensive silver paste.
In the above etching method (3), there are many problems on the management of the agents or an environmental point of view since the etching agent is used. The productivity (material yield) is low since it is necessary to dissolve the other part than the antenna. Further, the production speed is reduced since it takes time to dissolve it.